I have observed this strange behaviour with Windows (7 to 10). When active realtime protection is enabled, it blocks my own website. The page has nothing special (PHP and JavaScript), just good old plain HTML. The landing page (index.php) works, but after a while, some pages are blocked from loading. I don't see any error message, nor do I see any warning message from Windows. The site just is not loaded as if the content is zero bytes long.

Ping works, on other devices the site is loaded, it is only on Windows with realtime protection enabled that it is blocked (any browser, including curl). When I disable realtime protection, the pages are accessible.

On a newly installed system the site works great, but after some days it is blocked.

What browser/realtime protection/ other security software installed? What does a network sniffer show? Have you checked if there is an error on the server/php/the hosting provider?
– H. IddenDec 29 '15 at 12:40

No other security software installed (on some systems which I checked for reference). And yes I checked loggings on the server, there are no warnings or error, either on the PCs which blocks the loading of the webpage. All kind of browsers shows the same behavior. It does not depend on the browser, because I tried to load the URL with CURL and other command tools too.
– BrainDec 29 '15 at 12:51

What type of network are you on? Is there some kind of network firewall? Does your hoster use some kind of firewall/reverse proxy or mentions other security? Can you give examples of urls not working(do they contain special characters?)? Waht does wireshark show/at which stage is the connection rejected and how(while establishing the TCP-connection/sending the http-headers/sending answer/does it send an answer/closes the connection/stops sending packets)?
– H. IddenDec 29 '15 at 13:21

The blocked URLs can be accessed with the browser of a mobile phone via WLAN. There is no firewall active inside the router/modem. A iMac is also able to load the URLs without any problems. It seems to be related to the realtime protection of Microsoft Windows, because when I disable it the URLs can be loaded.
– BrainDec 29 '15 at 14:03